1. Field of the Invention
The present invention relates to an optical fiber used as an optical transmission line for an optical communication.
2. Description of the Related Art
As a conventional optical transmission line for an optical communication, a standard single-mode optical fiber (SMF) defined in the ITU-T (International Telecommunication Union Telecommunication Standardization Sector) G. 652 is most commonly used.
For an optical fiber used as an optical transmission line for a long-haul optical communication, a transmission loss far lower than that of the standard SMF is required to extend a transmission distance and to reduce a load to an optical amplifier as an optical repeater. In addition, a nonlinearity even lower than that of the standard SMF is required to suppress a quality degradation of an optical signal due to a nonlinear optical phenomenon and to make a transmission distance even longer. Furthermore, particularly for an optical fiber used in an optical fiber cable installed as a land transmission line, a low bending loss is required because a bending and a lateral pressure are applied on the optical fiber at the time of setting it in a slot of a cable.
As an optical fiber that realizes a low transmission loss, an optical fiber in which the core region is made of pure silica glass that does not contain germanium and the cladding layer is doped with fluorine and an optical fiber in which the core region is doped with chlorine and the cladding layer is doped with fluorine have been disclosed (see, for example, International Publication No. 00/42458 pamphlet). The pure silica glass mentioned above means a silica glass in which no dopant for controlling the refractive index is doped.
However, the convention optical fiber that realizes a low transmission loss requires a massive amount of fluorine at the time of manufacturing the fiber because the whole cladding layer is doped with fluorine. As a result, there has been a problem that the cost of the optical fiber is increased.
In addition, because a variety of technologies for using the standard SMF as an optical transmission line have been accumulated, an optical fiber that realizes low transmission loss and low nonlinearity while maintaining equivalent characteristics to those of the standard SMF is being required.